Language (linguistic) proficiency is the ability of an individual to communicate and/or perform their job in a specific language. Proficient speakers demonstrate both accuracy and fluency, and use a variety of discourse strategies.
For interpreters, language proficiency in two languages is a starting point; they also must possess interpreting skills that enable them to successfully convert meaning from one language into another.
There exist several reputable language proficiency scales:
ILR scale: The U.S. Interagency Language Roundtable descriptions of proficiency levels 0, 1, 2, 3, 4, and 5 characterize spoken-language use (http://www.govtilr.org/Skills/ILRscale1.htm).
ACTFL scale: Developed from the U.S. Federal Government’s ILR scale by the American Council on the Teaching of Foreign Languages, the ACTFL proficiency scale has four main levels (Novice, Intermediate, Advanced, Superior). The first three levels are each subdivided into three sublevels (Low, Mid, and High) (https://www.actfl.org/resources/actfl-proficiency-guidelines-2012).
CEFR scale: The Common European Framework of Reference for Languages: Learning, Teaching, Assessment is a guideline used to describe achievements of learners of foreign languages across Europe and, increasingly, in other countries. The CEFR distinguishes between four kinds of language activities: reception (listening and reading), production (spoken and written), interaction (spoken and written), and mediation (translating and interpreting). Four broad domains are distinguished: educational, occupational, public, and personal. A language user can develop various degrees of competence in each of these domains and to help describe them the CEFR has provided a set of six Common Reference Levels (A1, A2, B1, B2, C1, C2). (https://www.coe.int/en/web/common-european-framework-reference-languages/level-descriptions)
IELTS scale: The International English Language Testing System is an international standardized test of English language proficiency for non-native English language speakers. It is jointly managed by the British Council, IDP: IELTS Australia and Cambridge English Language Assessment. No minimum score is required to pass the test. An IELTS result or Test Report Form is issued to all test takers with a score from “band 1” (“non-user”) to “band 9” (“expert user”) and each institution sets a different threshold. (https://www.ielts.org/en-us/about-the-test/how-ielts-is-scored)
TOEFL scale: Test of English as a Foreign Language is a standardized test to measure the English language ability of non-native speakers wishing to enroll in English-speaking universities. TOEFL is scored on a scale of 0 to 120 points by adding scores from each of the four sections (Reading, Listening, Speaking, and Writing) which each receives a scaled score from 0 to 30. The test is accepted by many English-speaking academic and professional institutions; each institution establishes the minimally accepted score which varies from 61 to 111. (https://www.ets.org/toefl/institutions/scores/interpret/)